1. Field of the Invention
This invention relates to a thin-film temperature-sensitive resistor material used for thermometry or infrared detection and a process for producing the material.
2. Description of the Related Art
Thermometers and infrared ray sensors using a thin-film temperature-sensitive resistor material are conventionally known. FIG. 7 is a schematic view illustrating the structure of an infrared ray sensor which uses a thin-film temperature-sensitive resistor material. This sensor has a thin-film temperature-sensitive resistor material 8 between electrodes 11 on a supporting film 14. Over the material 8, a protective film 10 and an infrared absorption layer 9 are stacked, while an infrared reflective film 13 is disposed below the supporting film 14 via space 12. Infrared ray energy incident from the upper side of the drawing is converted to heat at the infrared absorption layer 9. The resulting increase of temperature causes a change in the resistance of the thin-film temperature-sensitive resistor material 8. Infrared rays can be detected by applying an electric current or voltage to the material 8 from the electrodes 11 and reading the change.
Metal oxides are mainly used as conventional thin-film temperature-sensitive resistor materials. Among them, a vanadium metal oxide (VMOx: M represents a metal and x stands for the oxidation number) which is an oxide with a transition metal vanadium (V), and another metal has been used as a material having a relatively high temperature coefficient of resistance (for example, Japanese Patent Laid-Open Nos. 253201/1985, 95601/1982 and 52882/1989).
As a conventional process for producing a thin-film temperature-sensitive resistor material, for example, a film can be formed on a substrate by carrying out sputtering with vanadium or vanadium and another metal as a target in an argon gas, followed by reaction with an argon-oxygen mixed gas. Alternatively, a film can be formed by the sol-gel processing in which a metal alcohol substance is applied to a substrate, followed by decomposition and removal of the organic residue by thermal treatment. FIG. 5 is a schematic view illustrating a process for producing a thin-film temperature-sensitive resistor material by the above-described conventional sputtering process. A desired thin-film temperature-sensitive resistor material can be formed over a substrate 5 by disposing, as illustrated in FIG. 5, the substrate 5 and target 6 in a sputtering chamber 4 equipped with gas inlet ports 1,2 and an exhaust port 7, introducing an argon gas from the gas inlet port 1 and an argon-oxygen mixed gas from the gas inlet port 2, and sputtering the target 6.
The performance of the thin-film temperature-sensitive resistor material is expressed by a temperature coefficient of resistance xcex1 (%/K) which is a resistance change rate per degree of a temperature change. FIG. 6 is a graph illustrating temperature characteristics of the resistance of the conventional thin-film temperature-sensitive resistor material composed of vanadium oxide, said material having been formed by sputtering vanadium in an argon-oxygen mixed gas. Temperature characteristics of the resistance of such a thin-film temperature-sensitive resistor material are also shown in FIGS. 1 and 2 on page 3 of Japanese Patent Laid-Open No. 253201/1985 or Jerominek, et al., Optical Engineering, 32, 2094 (1993), FIG. 1.
When used for the thermometry or infrared detection at the temperature (20 to 30xc2x0 C.) near room temperature, a thin-film temperature-sensitive resistor material is required to exhibit a high temperature coefficient of resistance around the room temperature. The thin-film temperature-sensitive resistor material as described above has, as shown in FIG. 6, a temperature coefficient of resistance as low as xe2x88x922%/K at about room temperature and is therefore insufficient in sensitivity.
When a material has a high temperature coefficient of resistance at about room temperature, more specifically, when the material has a temperature coefficient of resistance higher than xe2x88x922%/K at about room temperature as shown in Jerominek et al, Optical Engineering, 32, 2094(1993), FIG. 1, the specific resistance value of the material also becomes high, exceeding 0.1 xcexa9cm. Since it is generally impossible to form the thin-film temperature-sensitive material thicker than 1000 xc3x85, a high specific resistance value means nothing but a high resistance. The noise level generated in the material also becomes high. As a result, if the temperature coefficient of resistance at about room temperature is increased, the noise level is inevitably increased so that the sensitivity can not be improved.
As described above, it is difficult for the conventional thin-film temperature-sensitive resistor material composed of a metal oxide to simultaneously satisfy both the requirements for an increase in the temperature coefficient and lowering in the specific resistance value.
The present invention has been made with a view to overcoming the above-described problems of the prior art. An object of the present invention is to provide a thin-film temperature-sensitive resistor material which exhibits a high temperature coefficient of resistance at about room temperature (20 to 30xc2x0 C.) and a low specific resistance, and has excellent sensitivity at about room temperature; and a process for producing the material.
In one aspect of the present invention, there is thus provided a thin-film temperature-sensitive resistor material, which comprises, at a temperature-sensitive resistor portion thereof, a mixed crystal of a nitride and oxide of a transition-metal.
In another aspect of the present invention, there is also provided a process for producing the above-described thin-film temperature-sensitive resistor material, which comprises sputtering a transition metal target in a gas-atmosphere composed mainly of a nitrogen gas to form a mixed crystal of a nitride and oxide of a transition metal.
The present invention makes use of the properties that a transition metal oxide has both metallic and non-metallic properties and these properties separate into respective ones at a specific temperature as a border line; the electric properties of a metal nitride are easily controllable by a nitrogen content; a transition metal can be readily in the form of a mixed crystal of its oxide and nitride.
The transition temperature of a transition metal oxide from a metal to a non-metal is generally determined by the valence number of oxygen bound with the metal. In the present invention, this transition metal is in the form of a mixed crystal with a nitride which makes it possible to change the transition temperature more minutely according to a nitrogen/oxygen ratio bound with the metal. As a result, a thin-film temperature-sensitive resistor material which can simultaneously satisfy a high temperature coefficient of resistance and a low specific resistance at about room temperature is available.
Accordingly, the thin-film temperature-sensitive resistor material of the present invention simultaneously shows a high temperature coefficient of resistance and a low specific resistance value at about room temperature so that it is excellent in the sensitivity at about room temperature and is therefore very useful for the. applications such as infrared ray sensor and temperature detector each used at about room temperature.
Furthermore, according to the process of the present invention, such excellent thin-film temperature-sensitive resistor material can be easily and favorably produced.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate preferred embodiments and examples of the present invention.